9-11Research

Watch an animation of the Boeing 767 aircraft hitting the North Tower and the
rapid spread of the resulting fireball through the building.
Note how the vertical perimeter beams of the actual towers are represented by
horizontal lines in the animation. Anything to help sell the 'pancake theory'.

NOVA: So with the World Trade Center fire, the heat was much greater than might
have been expected in a typical fire?

Eagar: Right. We had all this extra fuel from the aircraft.
nearly all of which burned off in the impact fireballs and
within the first five minutes.
Now, there have
been fires in skyscrapers before. The Hotel Meridian in Philadelphia had a
fire, but it didn't do this kind of damage. The real damage in the World Trade
Center resulted from the size of the fire. Each floor was about an acre,
and the fire covered the whole floor within a few seconds.
There is no evidence that fires ever covered more than a half of any floor
in the South Tower.
Even the North Tower's fires were never as extensive as the Hotel Meridian's,
which gutted eight floors.
The South Tower's fires had dwindled, and were emitting only black smoke,
when it collapsed.
Ordinarily, it would
take a lot longer. If, say, I have an acre of property, and I start a brushfire
in one corner, it might take an hour, even with a good wind, to go from one
corner and start burning the other corner.

That's what the designers of the World Trade Center were designing for -- a
fire that starts in a wastepaper basket, for instance. By the time it gets to
the far corner of the building, it has already burned up all the fuel that was
back at the point of origin.
It's ridiculous to assert that the designers assumed a fire would only burn
on some portion of a floor at any one time.
So the beams where it started have already started
to cool down and regain their strength before you start to weaken the ones on
the other side.

On September 11th,
the whole floor was damaged all at once, and
that's really the cause of the World Trade Center collapse.
That is confident language to describe a "theory"
not supported by any evidence.
There was so much
fuel spread so quickly that the entire floor got weakened all at once,
whereas in a normal fire,
people should not think that if there's a fire in a high-rise
building that the building will come crashing down.
This was a very unusual situation, in which someone dumped 10,000 gallons of
jet fuel in an instant.
So why didn't the "collapse" occur when there was still some jet fuel left?

NOVA:
How high did the temperatures get, and what did that do to the steel
columns?

Eagar:
The maximum temperature would have been 1,600°F or 1,700°F.
It's impossible to generate temperatures much above that in most cases with
just normal fuel, in pure air.
In fact, I think the World Trade Center fire was
probably only 1,200°F or 1,300°F.
There is no evidence the fires were anywhere near that hot.
To be that hot they would have required good air supplies, which they did
not have as shown by the black smoke.
People who passed through the impact zone of the South Tower didn't report
the stairwells were particularly hot.

Investigations of fires in other buildings with steel have shown that fires
don't usually even melt the aluminum, which melts around 1,200°F. Most
fires don't get above 900°F to 1,100°F. The World Trade Center fire
did melt some of the aluminum in the aircraft and hence it probably got to
1,300°F or 1,400°F.
But more was melted than aluminum.
The dust contains abundant
iron-rich spheres,
which couldn't be produced by hudrocarbon-fueled office fires
but exactly match the residues of
aluminothermic explosives.
But that's all it would have taken to trigger the
collapse, according to my analysis.

NOVA:
You've pointed out that structural steel loses about half its strength at
1,200°F, yet even a 50 percent loss of strength is insufficient, by
itself, to explain the collapse.

Eagar:
Well, normally the biggest load on this building was the wind load,
trying to push it sideways and make it vibrate like a flag in the breeze. The
World Trade Center building was designed to withstand a hurricane of about 140
miles an hour, but September 11th wasn't a windy day, so the major
loads it was designed for were not on it at the time.

"You can't
explain the collapse just in terms of temperature."

As a result, the World Trade Center, at the time each airplane hit it, was only
loaded to about 20 percent of its capacity. That means it had to lose five
times its capacity either due to temperature or buckling -- the temperature
weakening the steel, the buckling changing the strength of a member because
it's bent rather than straight. You can't explain the collapse just in terms of
temperature, and you can't explain it just in terms of buckling. It was a
combination.

NOVA:
So can you give a sequence of events that likely took place in the
structural failure?

Eagar:
Well, first you had the impact of the plane, of course, and then this
spreading of the fireball all the way across within seconds. Then you had a hot
fire, but it wasn't an absolutely uniform fire everywhere. You had a wind
blowing
(or breeze?)
so the smoke was going one way more than another way, which means the
heat was going one way more than another way. That caused some of the beams to
distort, even at fairly low temperatures. You can permanently distort the beams
with a temperature difference of only about 300°F,
to a measurable degree in a laboratory.

NOVA:
You mean one part of a beam is 300°F hotter
than another part of the same beam?

Eagar: Exactly. If there was one part of the building in which
a beam had a
temperature difference of 300°F, then that beam would have become
permanently distorted at relatively low temperatures. So instead of being nice
and straight, it had a gentle curve. If you press down on a soda straw, you
know that if it's perfectly straight, it will support a lot more load than if
you start to put a little sideways bend in it. That's what happened in terms of
the beams.
He speaks of beams, but box columns supported the towers.
If the columns distorted enough to lose their rigidity,
it would have been apparent before the towers exploded.
They were weakened because they were bent by the fire.

But the steel still had plenty of strength, until it reached temperatures of
1,100° to 1,300°F.
Where is the evidence? At 1300° F,
the steel would have been glowing red hot
and would have been visible even in broad daylight.
In this range, the steel started losing a lot of
strength, and the bending became greater. Eventually the steel lost 80 percent
of its strength, because of this fire that consumed the whole floor.
The only time the fire had any chance to soften the steel was before the
jet fuel burned off.
Eagar makes the false implication that the softening of steel due to heating
is cumulative and irreversible.

If it had only occurred in one little corner, such as a trashcan caught on
fire, you might have had to repair that corner, but the whole building wouldn't
have come crashing down. The problem was, it was such a widely distributed
fire, and then you got this domino effect.
As if the columns of the building were merely balanced on top of each other,
Once you started to get angle clips
to fail in one area, it put extra load on other angle clips, and then it
unzipped around the building on that floor in a matter of seconds.
Not only is Eager's ridiculous zipper theory incompatible with the fact
that neither of the towers had fires covering an entire floor,
it is also relies on the deliberate omission of any mention
that there were cross-trusses, which were perpendicular to and
interlocked with the trusses shown in the animation.

Watch an animation of the floor trusses giving way,
followed by the buckling of the outer columns.

NOVA:
Many other engineers also feel the weak link was these angle clips, which
held the floor trusses between the inner core of columns and the exterior
columns. Is that simply because they were much smaller pieces of steel?
Those angle clips! Well, I guess we'll have to take Eagar's word that the
designers stupidly underengineered them, because the blueprints remain sealed
and unavailable for inspection by the public.
Actually they weren't "clips", they were brackets welded to the columns.

Eagar: Exactly. That's the easiest way to look at it. If you look at the whole
structure, they are the smallest piece of steel. As everything begins to
distort, the smallest piece is going to become the weak link in the chain.
dominoes, chain, ... how about house of cards.
They were plenty strong for holding up one truss, but when you lost several trusses,
the trusses adjacent to those had to hold two or three times what they were
expected to hold.

Those angle clips probably had two or three or four times the strength that
they originally needed. They didn't have the same factor-of-five safety as the
columns did, but they still had plenty of safety factor to have people and
equipment on those floors. It was not that the angle clips were inadequately
designed; it was just that there were so many of them that the engineers were
able to design them with less safety factor. In a very unusual loading
situation like this, they became the weak link.

NOVA: I've read that the collapse was a near free-fall.

Eagar: Yes. That's because the forces, it's been estimated, were anywhere from
10 to 100 times greater than an individual floor could support. First of all,
you had 10 or 20 floors above that came crashing down. That's about 10 or 20
times the weight you'd ever expect on one angle clip. There's also the impact
force, that is, if something hits very hard, there's a bigger force than if you
lower it down very gently.
That still doesn't address the fact the buildings fell at near free-fall acceleration.
If this were a gravity-driven collapse, the falls would have been greatly slowed by
two energy sinks: the crushing of each story encountered, and the acceleration of
its mass downward (since the stationary rubble would have to catch up with the falling top).
That energy could only have come from the kinetic energy of the falling mass,
decelerating it.
Even if you can buy the physical impossibility
that the floors by themselves could have fallen that fast,
it doesn't begin to explain where the energy to shatter the
outer wall and inner core columns came from.
And then there is the energy required to pulverize the floors' concrete into
fine dust before it hit the ground.
That energy alone is on the order of the entire potential energy of the
building's elevated mass.
This is proof that the buildings did not collapse of their own weight,
and Eagar has to know that.

NOVA:
Miraculously, a number of firefighters survived inside Tower One. They
were on the third or fourth floor in a stairwell, and immediately after the
collapse they looked up and saw blue sky above their heads -- their part of the
stairwell survived. How is that possible, with all the force of that
500,000-ton building coming down?
It's not possible -- it's a myth.
Even if it were possible, how could they have seen blue sky
when what was above them was an enormous dust cloud, over 500 feet high?

"They were very, very fortunate
that they happened to be in an area that was somewhat shielded."

Eagar:
Well, you have to understand the stairwells were reinforced areas of the
building. The stairwells were in the central core, which had more steel than
the outer areas, which were big open floors. So that extra steel formed a
little cage to protect them. It's still amazing, though.

Now, there could have been someone two floors below who could have been
completely crushed. It just depends on how the steel buckled.
It's interesting that examining the steel at gound zero to discover how this
buckling occurred was apparently not important enough to fund an investigation.
The most important evidence in the greatest structural failure in history --
the steel --
was hauled away and melted down with phenomenal speed.
If you take that
soda straw again, and you push it sideways, it will develop a buckle at some
location, probably somewhere in the middle third. Well, if you happen to be
where the buckling occurs, that area is going to get smashed, but if you're,
say, below where the buckling occurred, basically the whole thing can push
sideways. They were very, very fortunate that they happened to be in an area
that was somewhat shielded and protected by all the extra steel in the central
core.
This is the last part of the article that talks about the
how the buildings "collapsed".
So the steel in some places buckles, and those angle clips fail and
the floors start to pancake down.
Since the floors spanned the space between the core
and outer wall, the floors would slide down inside the building,
leaving the core and outer wall standing.
What then caused the steel box beams of the core and outer wall
to shread into one-story long pieces.
Where are the computer simulations of that?
How about an actual picture, like this of the
south tower just a few seconds into its "collapse"
(not in original article).
Does this look more like 'buckling' or 'exploding'?

I read one of those people's statements in the paper the other day, and he said
that if they'd been in the lobby, they'd be gone. I was in the lobby of the
World Trade Center years ago, and it was some three or four stories tall. What
was going to buckle? Well, the lobby had the longest columns, so they were
going to buckle. Those firefighters were just above that, so they were
protected by the buckling underneath, within this sort of steel cage.
In fact, that's how they design automobiles for crashworthiness. They try to
design the passenger compartment to be a cage, and the hood and trunk are
supposed to deform and absorb the energy so that you're protected by this
little cage of steel that hopefully won't deform.

Engineers have found evidence that the aluminum of the planes' fuselages
and wings may have melted, but there is no evidence that it burned.

NOVA:
There's a theory that the aluminum of the planes caught fire.

Eagar:
Yes, a number of people have tried to reinforce that theory. Now, the
aluminum of the planes would have burned just like a flare. Flares are made out
of aluminum and magnesium, so are fireworks, and they burn hot enough to melt
steel in certain cases.

However, they have had people sorting through the steel from the World Trade
Center, and no one has reported finding melted steel, which means that we
didn't have that aluminum flare. In any case, burning aluminum would have been
white-hot, about 4,000°F, and someone would have seen it even through that
dense black smoke.

Of course, aluminum can burn. That's what demolished the [British destroyer]
Sheffield in the Falklands War [when it was struck by an Argentinian
missile]. It wasn't the Exocet missile that destroyed the superstructure of the
Sheffield. The missile wasn't big enough, just like the plane wasn't big
enough to bring down the World Trade Center. That Exocet missile did damage the
Sheffield, but what doomed the Sheffield was the aluminum
superstructure caught fire and burned. So you suddenly had something like 1,000
or 10,000 times as much fuel as you had in that Exocet missile.

Now, this is not a type of fire we have to worry about in buildings. We
don't have anywhere close to those types of conditions. And we didn't have
those in the World Trade Center, in my opinion.

NOVA: How soon will a definitive report of the causes of the collapse be
released?

Eagar: Well, there's some very sophisticated analysis that various people in
the government, at universities, and at structural engineering firms are doing
to understand it. Most of those people have not yet published any conclusions.
To do a good job of research on something like this can typically take one to
two years. I don't expect to see any conclusive reports probably until about
the first anniversary of the attack.
Nearly four years and still waiting.
The truss theory was also promoted in FEMA's report.
It is hardly conclusive.
It provides misleading drawings of the towers' architecture,
and makes no attempt to explain the explosive ejections of dust from below
the "collapse zone", the shredding of the buildings' skeletons,
or the unimpeded falls of the towers' tops.

"There will still be people
worrying about this ten years from now."

There are different levels of analysis. You can do the back-of-the-envelope,
which was what I and other people did early on. But to do the full analysis
will take much longer.
I suspect there will still be people worrying about this ten years from now.
Like himself, since even though he doesn't seem to know much about structural engineering,
and doesn't seem too curious about the "collapse" or
concerned with the gaping holes in his own "theory,"
he might find his role in a cover-up disturbing.

NOVA: In your back-of-the-envelope analysis, you concluded the World Trade
Center was not defectively designed, but not everyone apparently accepts that
conclusion.

Eagar: A lot of people said, Well, the building failed. That's true, but
nothing is indestructible. The question is, why did it fail? In this case, as
I've explained, it was the fire covering the whole floor in a few seconds that
made this different from any other fire that anyone had ever designed for.

If people say, Well, couldn't we have designed it for this, I say, Yes, we
could have. We could build buildings that could survive a jet running into them
with a full fuel load.
And in fact the towers WERE designed to survive collisions by 707s carying
over twice the fuel that the similar-sized 767s were.
In fact, the military does. But they're bunkers. We
build these things for the President and the rest of the 150 leaders of the
country to go to as a secure area. You can do that, but your building costs go
up by a factor of about 100. Well, do we want to have 100 times fewer homes for
people to live in? Do we want to have 100 times fewer roads?

If we were to harden everything against a terrorist attack, we'd push ourselves
back into the first half of the 19th century in terms of living
style. Now, some people might consider that an improvement, but not everybody,
so society has some important tradeoffs here. There's got to be some middle
ground where we can make things more secure but not destroy our standard of
living.

NOVA: Anything we should do now to retrofit existing skyscrapers like the Sears
Tower?

Eagar: Well, one of the things that's really important and is relatively
inexpensive is a public communication system. I've been in high-rises when the
fire alarm goes off, and everyone looks around the room and decides, Should we
just continue meeting and ignore the fire alarm, or should we evacuate?
Fortunately, in most cases -- and I've had to be the person in a few of those
cases -- people say, Look, it's a fire alarm. We don't know if it's real.
Evacuate. So you need better public-address systems to inform people that this
is not a test, this is not a false alarm, you'd better get out of the
building.

Better communications systems may have
allowed more people to escape the towers before they collapsed, Eagar
believes. For instance, if more people had known that Stairway A
in the South Tower, shown here in green, had survived the impact,
more people may have gotten out before the building collapsed.

Survivors from the World Trade Center have said that some people took four or
five minutes to figure out there was something more than just some false alarm.
Other people started moving immediately. Obviously, the quicker people started
to move, the better chance they had of reaching safety.

NOVA: How about improving the fire safety of the building or putting in extra
stairwells?

Eagar: These are very difficult things to redesign into current buildings. They
can and will be added to future buildings. The simplest thing is the
communication system. And better training of firefighters. Those things will
definitely be done.

If you look at the World Trade Center disaster, it would have been greatly
minimized if the safety personnel had been aware of the danger they were in.
They didn't realize it was going to collapse.
Since it wasn't -- it was going to explode.
As I said earlier, there are only
a few engineers in the country who had ever designed skyscrapers like this who
would have realized, but they couldn't communicate within that first hour with
the people at ground zero. Nobody could call to New York City at that time.
Eagar is pleanty eager to speak for these designers.
So they had a change of heart after the planes hit,
suddenly realizing the scenario they had designed the towers to survive
would now cause them collapse? Was it seeing the smoke? --
"Holy ____ I forgot to factor in the fuel load!!!"
Eagar also ignores history:
There has never been a total collapse of a steel-framed high-rise
in history due to fire or any other cause except planned demolition
and severe earthquakes.
*.
* except for the 2001 Lower Manhattan Anomaly

So better communication. The military's known that for years. They've invested
tremendous amounts of money in better communications. That's been one of the
differences in having fewer lives lost on the American side in recent wars.
We've got much better C3I -- Command,
Control, Communications, and
Intelligence. They've spent billions of dollars, and it's saved thousands and
thousands of lives in the military. We can do that on the civilian side as well
for these big structures, though, in my opinion, skyscrapers are not the
problem anymore.

"A terrorist is not going to attack the things you
expect him to attack."

NOVA: What is?

Eagar:
I think the terrorist danger will be other things. A terrorist is not
going to attack the things you expect him to attack. The real problem is
pipelines, electrical transmission, dams, nuclear plants, railroads. A
terrorist's job is to scare people. He or she doesn't have to harm very many
people. Anthrax is a perfect example. If someone could wipe out one electrical
transmission line and cause a brownout in all of New York City or Los Angeles,
there would be hysteria, if people realized it was a terrorist that did
it.

Fortunately, we have enough redundancy -- the same type of redundancy we talk
about structurally in the World Trade Center -- in our electrical distribution.
We have that redundancy built in. I shouldn't say this, but this was how Enron
was able to build up a business, because they could transfer their energy from
wherever they were producing it into California, which was having problems, and
make a fortune -- for a short period of time.

NOVA: Gas pipelines don't have redundancy built in, though.

Eagar: No, but one advantage of a gas pipeline is the damage you can do to it
is relatively limited. You might be able to destroy several hundred yards of
it, but that's not wiping out a whole city. The bigger problem with taking out
a gas pipeline is if you do it in the middle of winter, and that gas pipeline
is heating 20 percent of the homes in the Northeast. Then all of a sudden you
have 20 percent less fuel, and everybody's going to have to turn the thermostat
down, and you're going to terrorize 30 million people.

The lesson we have to learn about this kind of terrorism is we have to design
flexible and redundant systems, so that we're not completely dependent on any
one thing, whether it's a single gas pipeline bringing heat to a particular
area or whatever.

Remember the energy crisis in 1973? That terrorized people. People were sitting
in long lines at gas pumps. It takes five or 10 years for society to readjust
to a problem like that. What happened in the energy crisis in 1973 was we had
essentially all our eggs in one basket -- the oil basket. But by 1983, electric
generating plants could flip a switch and change from oil to coal or gas, so no
one could hold a gun to our head like they did before.

Thomas Eagar is Thomas Lord Professor of Materials Engineering and Engineering
Systems at MIT. He was recently nominated to serve on a National Research
Council committee on homeland security. To see Eagar's article, "Why Did the
World Trade Center Collapse? Science, Engineering, and Speculation," which was
coauthored by MIT graduate student Christopher Musso, go to
www.tms.org/pubs/journals/JOM/0112/Eagar/Eagar-0112.htmlor read
this commented version.